This invention relates generally to RF and microwave superheterodyne receivers, and more particularly, the invention relates to such receivers employing photonic techniques including optoelectronic frequency translation.
Optoelectronic RF receivers are used to obtain high dynamic range, wide band width signal paths, high data rates, and low mass components. Such receivers have the ability to remotely locate various parts of the receiver system and to replace heavy, bulky, and stiff transmission lines with fiberoptic cables. Applications for the receivers are in satellite communications, electronic warfare, electronic support measures, and various radar applications.
FIG. 1 is a functional block diagram of a state of the art photonic superheterodyne receiver which includes an RF to photonic modulator 10 which modulates an optical signal from laser 12 with a received RF input signal 14. The RF signal is applied through a bias transistor circuit 16 with bias voltage from power supply 18 selected to maximize dynamic range and minimize harmonic distortion, for example. Modulator 10 converts the RF signal to optical sidebands of the CW optical carrier provided by the laser. A second modulator 20 in series with the RF signal and the sidebands of the optical carrier has a biased transistor power supply 22 and is driven by a local oscillator 24 to down convert the sidebands of the carrier to an IF frequency for detection by photodetector 26. A detected IF electrical signal is then applied to amplifier and low pass filter 28.
The RF to optical modulators and frequency translation devices may take the form of electroabsorption modulators, Mach-Zhender interferometers, and other conventional implementations. Narrow line width and low thermal drift lasers that can accommodate low RF frequency modulation are available using distributed feedback fiber laser technology or quantum dot laser technology.
Use of a frequency translation device such as modulator 20 in series with the RF signal path results in excessive RF loss, increased noise figure, and limited dynamic range. In a paper by Shin et al., “Optoelectronic RF Signal Mixing Using an Electroabsorption Waveguide as an Integrated Photo Detector/Mixer,” IEEE Photonics Technology Letters, Vol. 12, No. 2, February 2000, overcomes some of the limitations of the prior art circuit of FIG. 1. As shown in FIG. 2, Shin et al use two Nd:YAG lasers 30, 32 to generate a beat frequency at 900 MHz at the optical hybrid 34, which is the local oscillator signal in this circuit. The 900 MHz LO signal and a 1.0 GHz RF signal are applied to an electroabsorption (EA) waveguide 38 which functions as an integrated photo detector/mixer. RF signal 36 is applied through a RF circulator 40 and bias transistor circuitry 42 to modulator 38, and modulator 38 provides as an output the RF signal frequency fS, the local oscillator frequency fLO, and the sum and difference of the two frequencies fLO±fS. EA modulator 38 comprises an InAsP—GaInP multiple-quantum-well EA waveguide which provides frequency conversion of the radiofrequency signals through field controlled absorption. While this receiver eliminates a frequency translation device from the RF series signal path, thus reducing RF signal loss, the necessity for beating of two laser signals to provide a local oscillator signal requires very fine line width and some means of locking the two optical sources together to maintain local oscillator frequency stability. Also, it is difficult to remotely locate the receiver front end and deal with cables and LO distribution in actual implementation.
Kitayama, “Optical Down Conversion from Millimeter-Wave to IF-Band Over 50-km-Long Optical Fiber Link Using an Electroabsorption Modular,” IEEE Photonics Letters, Vol. 11, No. 2, February 1999 discloses another receiver in which an electroabsorption (EA) modulator is employed without the necessity for an RF mixer. As shown in FIG. 3, the Kitayama receiver again includes two lasers 42, 44 along with a fixed LO offset to generate the LO carrier signal at optical combiner 46. The outputs of the two lasers are applied to an optical combiner (photodiode) 48 with the beat frequency of the two lasers being applied to mixer 50 for mixing with a local oscillator signal. The resulting IF band is in the microwave region with the resulting signal being applied to a Mach-Zhender modulator 52 which modulates the optical field of laser 44. The output of MZM 52 is then combined with the output of laser 42 in optical combiner 46. The output of combiner 46 is then applied to EA modulator 52 along with the output of mixer 54 which receives the LO signal and the IF band (RF) signal. Again, like in the Shin receiver, an electroabsorption waveguide is used for demodulating the RF signal without the need for a further frequency translation device in the RF circuit path. However, like in Shin et al., two lasers are required in generating a carrier signal for use in the demodulation. This requires two locked phase or frequency lasers of narrow line width to produce a stable LO frequency.